Recently, the development for reducing the thickness of color image display apparatus has been carried out actively. Particularly, for example, Publication of Unexamined Japanese Patent Application (Tokkai-Hei) No. 3-67444 proposes a flat-type image display apparatus employing a beam scanning method in which the distance from a cathode to an anode is shortened significantly compared with a conventional cathode-ray tube (CRT) system. In the flat-type image display apparatus, a screen is divided into a plurality of sections vertically. An electron beam is deflected vertically to display a plurality of lines on each section. Further, the screen is also divided into a plurality of sections horizontally. In each section, phosphors of R, G, and B emit light sequentially. An amount of electron beams irradiated onto the phosphors of R, G, and B is controlled by the received color picture signals. Thus, a television picture is displayed as a whole.
In the above-mentioned flat-type image display apparatus, an electrode unit in which the distance from a cathode to an anode is shortened significantly and linear hot cathodes (hereafter referred to as "linear cathodes") as electron beam sources are housed in a flat-box type vacuum case. Electrodes forming the electrode unit are provided with small holes or slits for deflecting, focusing, and controlling electron beams emitted from the linear cathodes. The electron beams go through the electrodes while being controlled by the holes or slits in each electrode and accelerated to the anode to cause light emission of phosphors applied to the anode, thus displaying images.
FIG. 7 is an exploded perspective view showing the internal structure of a flat-type image display apparatus. In the flat-type image display apparatus, a back electrode 1, linear cathodes 2 (in the figure, only four linear cathodes are shown) extending horizontally, an electron beam extracting electrode 3, a signal electrode 4, focusing electrodes 5 and 6, a horizontal deflection electrode 7, and a vertical deflection electrode 8 are arranged sequentially. These sheet-like electrodes 3-8 are superposed via insulators and spacers, thus forming an electrode unit 11. The electron beam extracting electrode 3 is provided with electron beam extracting holes 12. Electron beams 13 emitted from the linear cathodes 2 are extracted through the holes 12 so as to form an apparent one electron beam per hole. An extracted electron beam 13 is controlled, focused, and deflected by the respective electrodes 4-8 to scan a subsection 14 on the anode screen.
The phosphors of R, G, and B are printed and applied onto screen sections, for example, 14-16 in the inner side of a front case 9 that is a flat-box type front glass case. Further, a metal-backed layer is formed on the sections 14-16 to apply high voltage. The electron beams are accelerated to have high energy and strike the metal-backed layer, thus exciting the phosphors so that the phosphors emit light. The electron beam 13 allows the subsection 14 of the screen to emit light to display a part of an image. Similarly, other electron beams cause light emission of all the other subsections, such as the subsection 16, to display images. Thus, a desired image is displayed on the whole screen. A rear case 10 and the front case 9 are combined and sealed, and then a vacuum is drawn on its inside, thus forming a flat-type image display apparatus.
FIG. 8 is a perspective view showing the appearance of a sealed flat-type image display apparatus. The front case 9 and the rear case 10 are baked to be sealed with low melting point glass. Numeral 17 indicates an exhaust pipe for drawing the vacuum inside the case, numeral 18 a high-voltage terminal of the anode, and numeral 19 outgoing terminals for controlling various electrodes forming the electrode unit. By connecting a driving circuit, a signal processing circuit, or the like to these terminals externally, the flat-type image display apparatus functions as a television receiver or a display unit.
Internal components constructing the aforementioned flat-type image display apparatus are exposed to high temperature repeatedly in a sealing step in the assembly and fabrication process and during operation of the apparatus as an image display apparatus. In other words, in the assembly and fabrication process, the apparatus is exposed to a high temperature of about 500.degree. C. both in fixing a plurality of fixing stands for attaching various electrodes to the glass rear case using low melting point glass and in combining and baking the front case and the rear case to seal the case using low melting point glass at a peripheral adhering portion of the case. Further, for example, a process of drawing high vacuum inside the glass case after sealing the glass case is carried out in a heating furnace at about 300-350.degree. C. Thus, the apparatus is heated repeatedly. During the operation of the apparatus as an image display apparatus, a number of linear cathodes stretched in a plane are heated to a high temperature of 600-700.degree. C. for generating electron beams. Due to such heat radiation, the various internal electrodes also are exposed to high temperature.
In order that a proper beam spot scans precisely the screen surface on which phosphors have been printed to avoid deviation of beam position on the screen so as to display vivid images with high precision even if the apparatus is exposed to high temperature in the aforementioned assembly and fabrication process and during the operation, the apparatus must have accuracy on a micron level and the accuracy must be maintained. However, generally objects exposed to high temperature repeatedly are subjected to thermal deformation such as expansion and contraction repeatedly due to the temperature change. Therefore, in order to allow the repeated exposure to high temperature and the maintaining of high accuracy to be compatible, problems of physically incompatible occurrences must be solved.
Particularly, while a flat-type image display apparatus has a flat shape, it is necessary to form the apparatus so as to have a glass-plate-like case body with a front case and a rear case, both of which are thick to have a thickness of about 10 mm, to obtain a resist pressure of the outside air by drawing high vacuum inside the case, thus causing extremely high thermal stress in the above-mentioned assembly process at high temperature.
The problems to be solved in a conventional example will be explained with reference to FIGS. 9 and 10 as follows.
FIG. 9 is a plan view showing an example of the arrangement of electrode support plates and electrode fixing plates for fixing an electrode unit including various electrodes that is attached to the inner face of a rear case of a conventional flat-type image displaying apparatus. In addition, FIG. 9 schematically shows a state in which thermal expansion and distortion occur in the above-mentioned heating processes.
FIG. 10 is a partial cross-sectional view showing a schematic structure of the flat-type image display apparatus shown in FIG. 9 in which the electrode unit is attached by fixing the electrode support plates and the electrode fixing plates that are assembled in a parallel-crosses form to fixing stands.
In FIG. 9, arrows A, B, C, . . . , P show thermal stress lines seen in a plane, and an alternate long and short dash line shows a slightly exaggerated distortion condition in which a rear case 10, electrode support plates 20, and electrode fixing plates 21 have been expanded and deformed due to the thermal effect as a result of the thermal stress.
Fixing stands 22 for fixing the electrode support plates 20 and the like to the rear case 10 are displaced according to the expansion and contraction of the glass-plate like rear case 10, which is not shown in the figure. The same is applied to the electrode unit to be fixed onto these electrode support devices.
The expansion caused by heating and the contraction caused by cooling may not be problems when all the components that are combined inside the case have the same coefficient of thermal expansion. However, in the conventional example, the case is made of glass, the electrode support plates 20 and the electrode fixing plates 21 are formed of a 50 Ni--Fe material, and a plurality of electrode plates forming an electrode unit 11 are made of an alloy (for instance, a 36 Ni--Fe alloy) having a low coefficient of thermal expansion. Therefore, the difference in coefficient of thermal expansion among those components causes the difference in distortion due to the thermal deformation. Consequently, cracks and warps occur at weak spots and stress concentration spots, which have been a problem.
FIG. 9 shows that the rear case 10 expands by being heated to the position shown with the alternate long and short dash line indicated with 10'. In this case, fixing stands 22a.about.22f tend to be displaced in the same way. An electrode unit (not shown in the figure) is fixed onto the fixing stands 22a.about.22d indirectly with fixing screws 23a-23d respectively. On the other hand, the electrode plates forming this electrode unit serve to control and supply focused electron beams and the electron beams must accurately strike R, G, and B phosphors that have been printed on the inner face of a front case 9 minutely. Therefore, when the screen and the electrodes are thermally displaced differently, basic performance as an image display apparatus cannot be obtained.
The electrode unit 11 made of a 36 Ni--Fe alloy is fastened and fixed to the electrode support plates 20 to form one component with screws 24a-24d for mounting the electrode unit in FIG. 9. The volume of deformation of the electrode unit itself is controlled to be small by employing an alloy that is not thermally deformed much. On the contrary, the electrode support plates 20 and the electrode fixing plates 21 are made of a 50 Ni--Fe alloy as mentioned above and thus have a coefficient of thermal expansion similar to that of glass. Therefore, the electrode support plates 20, the electrode fixing plates 21, and the rear case 10 are thermally deformed prior to the change of the electrode unit 11.
In addition, the thermal deformation caused by the difference in accuracy depending on the dimensional accuracy and assembly accuracy of the electrode support plates 20 and the electrode fixing plates 21 also must be considered.
In short, the intersection points of the electrode support plates 20 and the electrode fixing plates 21 that are assembled in a parallel-crosses form are fixed to the electrode unit made of a 36 Ni--Fe alloy using the screws 24a-24d for mounting the electrode unit so as to form one component. Therefore, the electrode support plates 20 and the electrode fixing plates 21 cannot be displaced at their intersection points and thus are displaced at their intermediate points in respective directions indicated with the arrows A, B, C, and D in a curved manner. Thus, the fixing stands 22e and 22f at the intermediate points are affected most and are the parts where cracks or the like occur easily.
FIG. 10 shows an example of such a state. The difference in displacement magnitude between the arrow S showing the thermal displacement direction of the electrode fixing plates 21 and the arrow R showing the thermal displacement direction of the rear case 10 leads to breakage and causes unwanted occurrence such as cracks 31 and the like in a low melting point glass 30, which has been a problem. In addition, such occurrence is not uniform, which also has been a problem.
When an external force such as vibration, impact from falling, or the like is applied, as shown in FIG. 10, the electrode support plates 20 serve as points of support since the electrode unit 11 contacts with and is supported by the electrode support plates 20 at both ends of the electrode unit 11. Thus, the electrode unit 11 resonates at a proper frequency to have the greatest amplitude in the vicinity of the central portion. In the worst case, the electrode unit 11 and linear cathodes 2 come into contact with each other and carbonates that have been applied to the linear cathodes 2 fall, which has been a problem.
For a plurality of linear cathodes, the electric fields at the central portion of the case and in the vicinities of the electrode support plates 20 are not uniform. Therefore, the difference in electron-beam emission capacity among the linear cathodes occurs and thus disturbs the uniformity in an image, which has been a problem.
Furthermore, in spattering a getter that adsorbs gases in a vacuum, the spattered getter adheres onto the linear cathodes and the outgoing terminals, thus causing bad insulation, which has been a problem.